Aliphatic isocyanates impart favorable ultraviolet-light stability to polyurethanes compared with the more conventional aromatic isocyanates. Unfortunately, aliphatic isocyanates are often difficult and costly to produce.
Aliphatic isocyanates are available from the reaction of aliphatic amines with phosgene. Phosgene is toxic, difficult to handle safely, and generates corrosive by-products. Non-phosgenation routes to isocyanates are therefore desirable.
U.S. Pat. No. 4,130,577, for example, teaches to prepare aliphatic isocyanates from the reaction of an alkali metal isocyanate with an alkyl halide precursor. Although high yields are possible, the slow rate of reaction and the halide by-product make this process and similar ones commercially unattractive.
An attractive route to aromatic isocyanates relies on thermal cracking of urethane precursors. When various reaction promoters are employed, the reaction rates and yields of isocyanates are favorable. Such processes are described in U.S Pat. Nos. 4,871,871, 4,873,364, and 4,883,908. A drawback of synthesizing isocyanates from urethanes is that the required urethane precursors are difficult to prepare from inexpensive, readily available feedstocks.
Krow et al. (J. Chem. Eng. Data 17 (1972) 116) describe the synthesis of benzyl urethanes from aryl ethers. A phenyl ether, such as anisole, is reacted with an equivalent of a 1,1-diurethane, such as methylene diurethane, in the presence of boron trifluoride etherate in refluxing toluene to give a benzylurethane; for example: ##STR1##
The poor-to-moderate yields reported (20-51%) are too low for the process to have commercial value. Also, there is no indication that the aromatic ring can be di- or polyalkylated by this method. The ability to introduce more than one urethane group on an aromatic ring is important because it allows for the synthesis of precursors to useful di- and polyisocyanates.
Merten et al. (Belgian Patent No. 627,280) teach (Example 5) that the reaction of phenol with 3.1 equivalents of methylene-bis-(ethyl carbamate) in toluene at 100.degree. C. gives 2,4,6-tris(ethyl carbamylmethyl)phenol.
Singh et al. (U.S. Pat. No. 4,879,410) teach a process for preparing aralkyl mono- and diurethanes or ureas by carbamylmethylation or acid-catalyzed addition at 40.degree. C.-b 100.degree. C. of formaldehyde and esters of carbamic acid to aromatic hydrocarbons. The process comprises heating an aromatic hydrocarbon, a carbamate such as methyl carbamate, formaldehyde, and an acid catalyst to form the aryl-substituted alkyl urethane. The preferred catalysts are sulfuric and phosphoric acids. In general, the "catalyst" is present in excess and also serves as a solvent. Other optional solvents taught include protic solvents such as methanol, ethanol, and acetic acid, and halogenated hydrocarbons such as dichloromethane, ethylene dichloride, and chlorobenzene.
The key advantage of the Singh process is that valuable isocyanate precursors can be prepared from relatively inexpensive aromatic hydrocarbons. The process, however, suffers from several drawbacks: (1) Mineral acid solvents are commercially undesirable because they are corrosive and extremely costly to neutralize and dispose of properly; (2) Yields of the dicarbamates are typically quite low (about 10-45% with m-xylene, for example); and (3) A large proportion of the aromatic hydrocarbon is not converted to useful products.
A commercially viable non-phosgene route to aliphatic isocyanates is needed. Preferably, the process uses inexpensive, readily available feedstocks. Especially desirable is a process for preparing high yields of aliphatic urethane compounds (aliphatic isocyanate precursors), especially those with more than one urethane group in the molecule, without the corrosivity, waste treatment, and product-loss problems associated with the use of mineral acid solvents.